The etiology and pathogenesis of viral gastrointestinal and respiratory tract infections in man are being examined. A virus particle morphologically similar to but antigenically distinct from the Norwalk agent has been associated with a waterborne outbreak of gastroenteritis, and the disease has been transmitted to normal volunteers. Cell-mediated immune studies of influenza infection and vaccination in man indicate that both antigenically specific and nonspecific cytotoxic responses are seen. Patients with systemic lupus lack cytotoxic responses after vaccination and have variable humoral antibody responses. Clinical trials of monovalent and trivalent inactivated vaccines containing influenza A/USSR/90/77 (H1N1) antigens in volunteers, primarily over 24 years age, resulted in low reactogenicity and high rates of sero-responses.